The present invention relates to micro-machined electromechanical systems, and more particularly to a MEMS vibratory gyroscope having electronic tuning and alignment.
Micro-gyroscopes are used in many applications including, but not limited to, communications, control and navigation systems for both space and land applications. These highly specialized applications need high performance and cost effective micro-gyroscopes.
There is known in the art a micro-machined electromechanical vibratory gyroscope designed for micro-spacecraft applications. The gyroscope is explained and described in a technical paper entitled xe2x80x9cSilicon Bulk Micro-machined Vibratory Gyroscopexe2x80x9d presented in June, 1996 at the Solid State Sensors and Actuator Workshop in Hilton Head, S.C., and in U.S. Pat. No. 5,894,090.
A typical gyroscope has a resonator having a xe2x80x9ccloverleafxe2x80x9d structure consisting of a rim, four silicon leaves, and four soft supports, or cantilevers, made from a single crystal silicon. The four supports provide mechanical support and restoring force for the harmonic motion of the structure. A metal baton is rigidly attached to the center of the resonator, in a plane perpendicular to the plane of the silicon leaves, and to a quartz base plate spaced apart from the silicon leaves. The quartz base plate has a pattern of electrodes that coincides with the cloverleaf pattern of the silicon leaves. The electrodes include two drive electrodes and two sense electrodes.
The micro-gyroscope is electrostatically actuated and the sense electrodes capacitively detect Coriolis induced motions of the silicon leaves. The micro-gyroscope has a low resonant frequency due to the large mass of the metal post and the soft cantilevers. The response of the gyroscope is inversely proportional to the resonant frequency. Therefore, a low resonant frequency increases the responsivity of the device.
The cloverleaves provide large areas for electrostatic driving and capacitance sensing. Applying an AC voltage to capacitors that are associated with the drive electrodes excites the resonator. This excites the rotation Tx about the drive axis and rocking-like displacement Ty for the leaves.
Because the post is rigidly attached to the leaves, the rocking movement of the leaves translates to movement of the baton. When the leaves oscillate in the drive mode, the displacement of the post is near parallel to the leaf surface in the y-direction. When a rotation rate is applied about the z-axis, Coriolis force acts on the oscillating post and causes its displacement in the x-direction. The baton displacement is translated back into the rocking motion, Ty, of the leaves. The baton provides large Coriolis coupling that transfers energy between the two orthogonal rocking modes.
The control electronics associated with the micro-gyroscope include an actuation circuit that is essentially an oscillator around the micro-gyroscope that locks onto the drive resonance mode. The signals from the sense electrodes are summed to remove the differential signal between them and the response of the sense resonance from the feedback loop. On the other hand, the sense circuit subtracts the signals from the sense electrodes to remove the common-mode drive signal.
Micro-gyroscopes are subject to electrical interference that degrades performance with regard to drift and scale factor stability. Micro-gyroscopes often operate the drive and sense signals at the same frequency to allow for simple electronic circuits. However, the use of a common frequency for both functions allows the relatively powerful drive signal to inadvertently electrically couple to the relatively weak sense signal.
Typically, prior art micro-gyroscopes are open loop and untuned. If the drive frequency is tuned closely to a high Q sense axis resonance, large mechanical gain and low sensitivity to sensor noise is possible. High Q also results in low rate drift.
However, close tuning leads to large uncertainty in the gain and phase of the open-loop response. Phase variations lead to added rate drift errors due to quadrature signal pickup and the gain variations lead to rate scale factor errors. Operating the open-loop micro-gyroscope in a closely tuned manner results in higher scale factor error, higher rate errors due to mechanical phase shifts, and slower response with sensitive lightly damped resonances. Additionally, the response time of the open-loop micro-gyroscope is proportional to the damping time constant, Q, of the sense resonance. To reduce rate drift, very long natural damping time constants are required, slowing the response time.
If the drive frequency is tuned closely to a high Q sense axis resonance, a force-to-rebalance method that incorporates complex demodulators and modulators in multiple re-balance loops is necessary. The modulators and demodulators provide coherent feedback only for signals modulating the drive frequency, and therefore do not provide active damping of independent sense resonance vibrations. These vibrations, if not exactly matched to the drive frequency, are not actively damped resulting in false rate signals or noise.
Noise and drift in the electronic circuit limit micro-gyroscope performance. Therefore, prior art micro-gyroscopes perform poorly and are unreliable in sensitive space applications. Previous open loop operation is intentionally split between two rocking mode frequencies. Therefore, the rocking mode axes tend to align with the spring axes and electrode sense and control axes.
Closed loop control enables close tuning of the rocking modes. However, residual imbalances result in non-alignment of rocking mode axes with electrode axes. This produces a large quadrature error signal and second harmonics on the output axis sensor which limits the amount of amplification and closed loop gain that can be applied. The large quadrature error signal also causes false rate indications due to phase errors in the demodulation. The lack of tuning of the two modes due to mismatch of the spring reduces the sensor mechanical gain and increases rate noise.
The present invention is a method for electronically aligning the sense and control axes with the modal axes and tuning the drive frequency to the sense mode frequency by adjusting the AGC loop phase. Cloverleaf micro-gyroscopes typically employ a control circuit that is electrically coupled to the electrodes. The circuit provides drive signals to drive electrodes to oscillate the leaf structure and to receive a sensing signal from the sensing electrodes to detect response of the oscillating leaf structure to external physical phenomena.
In the present invention, an electronic coordinate transformation is performed on the control voltages and on the sense voltages using an attenuator network, electronically aligning the drive and sense axes of the micro-gyroscope.
The micro-gyroscope is electronically tuned by virtue of design symmetry and precise manufacture. However, slight temperature or pressure variations may lead to tuning or alignment variations and bias drift. The present invention provides electronic compensation based on quadrature error to correct for these variations.
It is an object of the present invention to improve the aligning and tuning of rocking mode Coriolis micro-gyroscopes. It is another object of the present invention to electronically align the sense and control axes by way of electronic adjustments.
It is a further object of the present invention to tune the drive and sense mode frequencies by adjustment of the AGC loop.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.